Back Door or Trap Door - Computer Definition

A software bug or some undocumented software
feature that a cracker leaves behind, after exploiting a system, to be able to
reenter at a later point in time. Note, however, that back or trap doors can be
a function of poor software design; that is, during its development, a
programmer may have built in a software bug that was not removed when the
software was put in production. The unwitting consumer who purchases the
software becomes, in a sense, a target-in-waiting for a crack attack.

Back doors try to evade conventional clean-up methods by
system administrators, such as ongoing changes to passwords, cleaning of the registry/configuration
files, and the removal of suspicious software. Moreover, back doors tend to
evade logging procedures; thus, even though every incoming connection to a
system is supposedly logged, chances are that the back door provides a means of
logging in without being logged.
Finally, back doors are covert in the real sense that they hide well. Even if
the system administrator scans a
system looking for suspicious software, chances are the back door has used
techniques capable of missing the scan.

One more essential point about back doors is this: Users of
computer systems are, in large part, the cause of their own cracking
misfortunes. Although most computers today allow BIOS
passwords (the software that first runs when the computer starts) to be set to
prevent the booting of the computer without an administrator’s first typing the
password, because so many users lose or forget their passwords, BIOSes
frequently have back door passwords to permit the legitimate password to be set. Furthermore, much remote
network equipment such as routers, switches, and dial-up banks have back doors
for remote telnet.